1. Field of the Invention
This invention relates to a method of determining the longitudinal reference axis of an object. The method is particularly suitable for use with a computer software module that finds the long axis of a tooth as part of an orthodontic diagnosis and treatment analysis.
2. Description of the Related Art
The longitudinal axis, or long axis, of objects that are relatively simple in shape is often easy to determine. For example, the long axis of an object having a uniform circular or rectangular shape in transverse cross-sections can be derived by extending a reference line perpendicularly through the center of the cross-sections. However, the long axis of an object that is complex in shape may be substantially more difficult to determine.
For example, teeth are generally considered to be complex in shape. The exposed portion of each tooth, also known as the clinical crown, varies widely from tooth to tooth. For instance, incisal teeth generally taper to an outer edge resembling a chisel blade while cuspid teeth generally taper to a point, called a cusp. Bicuspid teeth have two outer cusps that are separated by a recess, while molar teeth typically have four cusps.
Additionally, the roots of the teeth vary from one tooth to another. Anterior and cuspid teeth typically have one root while bicuspid teeth often have two roots. The molar teeth usually have three roots.
Moreover, it is known that the shape of teeth can vary widely from one patient to the next. While teeth generally have certain common characteristics (for example, the cuspid teeth typically have a single root and taper to an outer occlusal point), the exact shape of a cuspid tooth can vary considerably from one patient to another when closely inspected.
In the field of dentistry, there is often a desire to establish the long axis of one or more teeth in order to facilitate diagnosis and/or treatment. For example, the field of orthodontics is concerned with repositioning and aligning the patient's teeth for improved occlusion and aesthetic appearance. The long reference axis (longitudinal axis) of a tooth can serve as a convenient shorthand description for identifying the tooth's actual or desired orientation.
Establishing the long axis of the tooth provides other benefits for the orthodontic practitioner as well. For example, orthodontic treatment often involves the use of tiny slotted appliances, known as brackets, that are fixed to the patient's anterior, cuspid, and bicuspid teeth. An archwire is received in the slot of each bracket and serves as a track to guide movement of the teeth to desired orientations. The ends of the archwires are usually received in appliances known as buccal tubes that are secured to the patient's molar teeth.
A number of orthodontic appliances in commercial use today are constructed on the principle of the “Straight Wire Concept” developed by Dr. Lawrence F. Andrews, D.D.S. In accordance with this concept, the shape of the appliance, including the orientation of the slots of the appliances, is selected so that the slots are aligned in a flat reference plane at the conclusion of treatment. Additionally, a resilient archwire is selected with an overall curved shape that normally lies in a flat reference plane.
When the archwire is placed in the slots of the straight wire appliances at the beginning of orthodontic treatment, the archwire is often deflected upwardly or downwardly from one appliance to the next in accordance with the patient's malocclusions. However, the resiliency of the archwire tends to return the archwire to its normally curved shape that lies in a flat reference plane. As the archwire shifts toward the flat reference plane, the attached teeth are moved in a corresponding fashion toward an aligned, aesthetically pleasing array.
As can be appreciated, it is important for the practitioner using straight wire appliances to fix each appliance in the exact proper position on the corresponding tooth. If, for example, the bracket is placed too high in an occlusal direction on the tooth surface, the archwire will tend to position the crown of the tooth too close to the gingiva (gums) at the end of treatment. As another example, if the bracket is placed to one side of the center of the tooth in a mesial-distal direction, the resultant tooth orientation will likely be an orientation that is excessively rotated about its long axis.
As a consequence, practitioners in the past have often taken considerable care when bonding straight wire appliances to the patient's teeth to ensure that the appliances are precisely positioned on the teeth at correct locations. Some practitioners prefer to place each appliance on the location that is known as the facial axis point of the tooth. Unfortunately, visual determination of the facial axis point is often difficult to carry out with precision and may be subjective in nature.
In theory, the facial axis point of the tooth crown is defined as the intersection of the mid-transverse plane, the mid-sagittal plane, and the facial surface of the clinical crown. The mid-sagittal plane is a reference plane that includes the long axis of the tooth and separates the mesial and distal halves of the clinical crown. The mid-transverse plane of the crown is perpendicular to the long axis of the tooth and separates the occlusal and gingival halves of the clinical crown. In practice, however, such a determination is difficult to carry out with a high level of accuracy when a visual assessment is employed.
In recent years, there has been increased interest in the use of digital microcomputers and software for orthodontic diagnosis and treatment. For example, placement of orthodontic brackets using either direct bonded or indirect bonded techniques can be carried out with much greater precision using microcomputers and robotics than can be accomplished by visual placement techniques. Clearly, such increased placement accuracy of orthodontic appliances significantly increases the likelihood that the teeth will be properly positioned at the conclusion of orthodontic treatment.
Digital microcomputers and software are also highly useful in treatment diagnosis and planning. For example, data representing the shape and orientation of the patient's teeth may be processed by a microcomputer to help predict the appearance of the teeth at the conclusion of treatment or at various stages during the course of treatment. This data may also be used to predict the path of movement of the teeth as treatment progresses.
Presently, there is a need in the art to have an automated method for determining the long axis of an object that is complex in shape, such as a tooth. Preferably, the method would eliminate subjective factors that have typically been associated with certain visual long axis determinations in the past. Further, any such method should be usable in the field of dentistry with any desired tooth without regard for the number of roots or the shape of the clinical crown.